Activated T lymphocytes play critical roles in host defense against viral infection and cancer and are involved in the progression of autoimmune diseases. Two distinct signals are typically required for optimal activation of antigen-specific T lymphocytes. The first signal is provided by the interactions of the complex of antigenic peptide and major histocompatibility complex (MHC) with the T cell receptor (TCR). The second signal is delivered to T cells by costimulatory molecules expressed on antigen-presenting cells (APCs). Studies have demonstrated that costimulatory interactions stimulate T cell growth, up-regulate cytokine production, and promote T cell differentiation. Furthermore, ligation of costimulatory molecules provides a survival signal essential for T cells to prevent apoptosis. In addition, costimulatory signals appear to be critical for the induction and maintenance of T cell anergy. More importantly, there is clear interest in therapeutic manipulation of costimulatory pathways because the control of costimulatory signals can provide a means either to enhance or to inhibit immune responses.